Tuesday, September 2, 2014

Thiomargarita namibiensis is a gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is one of the largest bacteria ever discovered, as a rule
0.1–0.3 mm (100–300 µm) in diameter, but sometimes attaining 0.75 mm (750 µm). Cells
of Thiomargarita namibiensis are large enough to be visible to the naked eye. Although the species holds the record for the most massive bacterium, Epulopiscium fishelsoni – previously discovered in the gut of surgeonfish – grows slightly longer, but narrower.

Thiomargarita means "sulfur pearl". This refers to
the appearance of the cells; they contain microscopic sulfur granules that scatter incident
light, lending the cell a pearly lustre. Like many coccoid bacteria such as Streptococcus their cellular division tends to occur along a single axis, causing their cells to form chains, rather like strings of
pearls. The species name namibiensis means "of Namibia".

OCCURRENCE
The species was discovered by Heide N. Schulz and others in 1997, in the coastal seafloor sediments of Walvis Bay (Namibia). Schulz and her colleagues, from the Max Planck Institute of Marine Biology,
were on a Russian research vessel, the Petr Kottsov,
when the white color of this microbe caught their interest. They were actually looking for other recently found sulfide-eating marine bacteria, Thioploca and Beggiatoa. In the end they ended up with an entire new discovery, of a much larger
cousin strain of the two other bacteria. In 2005, a
closely related strain was discovered in the Gulf of
Mexico. Among other differences from the Namibian strain, the Mexican strain does not seem to divide along a single axis and accordingly does not form chains. No other species in the genus
Thiomargarita are known at present.

The previously largest known bacterium was Epulopiscium fishelsoni, at 0.5 mm long.[5]
Distribution of

STRUCTURE:
Although Thiomargarita namibiensis is closely related to Thioploca and Beggiatoa in function, their structures proved to be vastly different. Thioploca
and Beggiatoa cells are much smaller and grow tightly stacked on each other in long filaments.
Their shape is necessary for them to shuttle down into the ocean sediments to find more sulfide and nitrate. In contrast, Thiomargarita grow in rows of
separate single ball shaped cells, not allowing them to have the range of mobility that Thioploca and Beggiota have. With their lack of movement they
have adapted by evolving very large nitrate-storing
bubbles, called vacuoles, allowing them to survive
long periods of nitrate and sulfide starvation. The
vacuoles give Thiomargarita the ability to stay immobile, just waiting for nitrate rich waters to sweep over them once again. These vacuoles are what account for the size that scientists had
previously thought impossible. Scientists disregarded large bacterium because bacteria rely
on diffusion to move chemicals around, a process that works only over tiny distances. This implies
that the cytoplasm has to be close to the cell wall, greatly limiting their size. But Thiomargarita are an exception to this size constraint as their cytoplasms form along the peripheral cell membrane, while the nitrate-storing vacuoles occupy the cells of the
Thiomargarita. As these vacuoles swell they greatly contribute to the record holding size. It holds the record for the world's largest bacteria with a volume
three million times more than that of the average
bacteria.

METABOLISM:
The bacterium is chemolithotrophic, and is capable
of using nitrate as the terminal electron acceptor in the electron transport chain. The organism will oxidize hydrogen sulfide (H2S) into elemental sulfur
(S). This is deposited as granules in its cytoplasm
and is highly refractile and opalescent, making the organism look like a pearl.
While the sulfide is available in the surrounding sediment, produced by other bacteria from dead microalgae that sank down to the sea bottom, the nitrate comes from the above seawater. Since the bacterium is sessile, and the concentration of available nitrate fluctuates considerably over time, it stores nitrate at high concentration (up to 0.8
molar) in a large vacuole, like an inflated balloon,
which is responsible for about 80% of its size.
When nitrate concentrations in the environment are low, the bacterium uses the contents of its vacuole for respiration. Thus, the presence of a central
vacuole in its cells enables a prolonged survival in sulfidic sediments. The non-motility of
Thiomargarita cells is compensated by its large
cellular size.
Recent research has also indicated that the bacterium may be facultatively anaerobic rather
than obligately anaerobic, and thus capable of respiring with oxygen if it is plentiful.

SIGNIFICANCE:
Gigantism is usually a disadvantage for bacteria.
[11] Bacteria obtain their nutrients via simple diffusion process across their cell-membrane, as they lack the sophisticated nutrient uptake
mechanism found in eukaryotes. A bacterium of large size would imply a lower ratio of cell
membrane surface area to cell volume. This would limit the rate of uptake of nutrients to threshold levels. Large bacteria might starve easily unless
they have a different backup mechanism. T. namibiensis overcomes this problem by harboring large vacuoles that can be filled up with life-
supporting nitrates.